Atomic
spectroscopy has experienced remarkable growth and diversity in the past
few years, making it more difficult for analysts to keep up with developments
in the field. Atomic spectroscopy is not one but three techniques: I) atomic
absorption, 2) atomic emission, and 3) atomic fluorescence. The first two
are the most common and widely used techniques. The linear relationship
between the amount of light absorbed or emitted and the amount of species
of interest is called the Beer-Lambert Law. It can be used to find unknown
concentrations by measuring the light emitted or absorbed.

1) Atomic
Absorption is the process where vaporized atoms absorbs light and
is measured.The
basic instrument for atomic absorption requires a light source, an atom
source, a monochrometer to isolate the specific wavelength of light, a
detector, some electronics to treat the signal, and a data display. The
light source is usually a hollow cathode lamp.

2) Atomic
Emission is a process in which the light emitted by excited atoms or
ions is measured.

The
basic instrument for atomic emission is similar to atomic absorption except
that it has no primary light source. The critical component in emission
is the atomization source because it must provide all the energy to excite
as well as atomize the atoms. Previously many sources were tried but the
ICP eliminates most all problems associated with past emission sources.
This has revolutionized the utility of atomic emission spectroscopy.The ICP is an argon plasma maintained by the
interaction of an RF field and ionized argon gas. The ICP can reach temperatures
around 10,000 K with sample temperatures between 5,000 and 8,000 K. These
temperatures allow complete atomization of elements thus minimizing chemical
interference effects.
In ICP-MS, the function of the Mass Spectrometer is similar to that
of the monochrometer in Atomic Absorption or ICP Emission systems. In ICP-MS,
rather than separating light into wavelengths, the mass analysis separates
the ions from the ICP according to their mass/charge ratio. The ICP-MS
combines the multielement capabilities and broad linear range of ICP with
the exceptional detection limits of graphite furnace AA.

Selecting
the Proper Atomic Spectroscopy Technique

There
are four techniques normally Suited for analytical determinations and they
are:

1) Flame Atomic Absorption

2) Graphite Furnace Atomic
Absorption

3) Inductively Coupled
Plasma Emission

4) Inductively Coupled
Plasmal Mass Spectrometry

A
clear understanding of the analytical problems and the capabilities provided
by the different techniques is necessary. Some important criteria for selecting
a particular technique include 1) detection limits, 2) analytical working
range, 3) sample throughput, 4) cost, and 5) ease of use.

Detection Limits

Without
adequate detection limit capabilities, lengthy preparation may be required
prior to analysis. Typical detection limits for the major atomic spectroscopic
techniques are shown in Table 1. Generally, the best detection limits are
attained using ICP-MS and Graphite Furnace Atomic Absorption as shown below.

Analytical Working Range

The
analytical working range is just the concentration range over which the
quantitative results can be obtained without having to recalibrate the
system (the linear relationship between amount of light measured and concentration).
For the GFAA there is approximately 2 order of magnitude for its working
range where the ICP-AES has a 5th order of magnitude as its working range
and the ICP-MS has a 6th to 8th order of magnitude as its working range.

All detection limits are
given in micrograms per liter and were determined using elemental
standards in dilute aqueous solution. All detection limits are based on
a 98% confidence
level (3 standard deviations).
Atomic absorption and ICP emission detection limits were determined using
instrumental parameters optimized for the individual element. ICP
emission detection limits obtained during multielement analyses will typically
be within a
factor of 2 the values shown.
Cold vapor mercury detection limits were determined with a FIAS-200 flow
injection
system with amalgamation accessory. Hydride detection limits were determined
using a
MHS-10 Mercury/Hydride system. Fumace AA (Model 5100 Pc with 5100 ZL Zeeman
furnace Module or Model 4100 ZL) detection limits were determined using
STPF conditions
and are all based on 20 microliterL sample volumes and use of a L'vov platform.
ICPMS detection limits were determined using an ELAN 5000. Letters following
an ICP-MS
detection limit value refer to the use of a less abundant mass for the
determination as
follows:a-C 13 b-Ca 44 c-Fe 54, d-Ni 60, e-S 34, f-Se 82

Sample
ThroughputSample throughput is the number of samples which
can be analyzed or elements to be determined per unit
time. Analyses near the limit of detection or where absolute precision
is required are more time consuming
than other less demanding analyses.GFAA is basically a single element technique because of the need
~ thermally program the system
to remove solvent and matrix components prior to atomization. The GFAA
has a relatively low sample
throughput where a typical determination (single burn) requires 2-3 minutes
per sample per element.ICP Emission is a true multielement technique with exceptional sample
throughput. IC? emission
systems typically can determine l~80 elements per minute in individual
samples. For few elements the ICP
is limited by the time needed to equilibrate the plasma which is typically
15-30 seconds (0.25-0.5 minutes)ICP-MS has the same multielement capabilities and time requirements
as IC? but can get much
better detection limits like those in GFAA.
To run one sample by GFAA it takes 4 minutes to precondition the graphite
tube, 8 minutes to calibrate
using a four point calibration method. Another 8 minutes are taken up by
internal analysis checks and
blanks. that is 22 minutes just for calibration without any complications.
To analyze the unknown sample
takes 2-3 minutes providing it is in the right working range or it will
need to be diluted which takes still
more time. And to do eight elements it will take approximately 4 hours
(240 minutes). On the ICP it takes
10 minutes to calibrate, in a typical case 23 elements, and then l minute
per burn but all 23 elements are
determined simultaneously. That is a total of 11-12 minutes. That is 20
times faster and there is three times
more information.

Cost
Instrumentation for single element atomic spectroscopy (flame AA and GFAA)
is generally less costly than
that for the multielement techniques such as ICP and ICP-MS as the former
are less complex systems There
is cost differentials among instruments for the same technique. Instruments
which offer versatility frequently
offer a greater degree of automation than that of the basic instrument.
Cost of the instrument is only one
issue to look at regarding the total cost of what the data determines.
Other things may include cost of down
time, the value of real time data in solving critical excursions, cost
of sample delivery. and cost of
maintenance.

Ease of UseOther comparison criteria for analytical techniques
include ease of use, operator skill level, and availability
of documented methodologies.

GFAA applications are well documented. GFAA has exceptional detection
limit capabilities but
with a limited working range. Sample throughput is less than that of other
atomic spectroscopy techniques.
Operator skill requirements are more than minimum expertise to obtain best
results, especially on the more
difficult samples we encounter.

ICP Emission is the best overall multielement atomic spectroscopy
technique with excellent
sample throughput and very wide analytical range. Good documentation is
available for numerous
applications. Operator skill requirements are intermediate.

ICP-MS is a relatively new technique with exceptional multielement
capabilities at trace and
ultratrace concentration levels. Isotopic determinations can also be performed
on the ICP-MS. Good basic
documentation for interferences exists. Application documentation is limited
but growing rapidly. ICP-MS
requires operator skill similar to those for ICP and GFAA.SummaryMost often the selection of technique is based on
analyte concentrations, flame AA and ICP emission are favored
for moderate to high levels while graphite furnace AA and ICP-MS are favored
for low levels. ICP and
ICP-MS are multielement techniques favored where large numbers of samples
are to be analyzed.